A hydraulic-type valve timing control device is known in the art. A conventional valve timing control device of this kind has a housing rotor and a vane rotor, each of which is rotated in its circumferential direction in synchronism with rotation of a crank shaft and a camshaft of an engine. The valve timing control device adjusts a rotational phase of the vane rotor with respect to the housing rotor (hereinafter simply referred to as the rotational phase). In the valve timing control device, an inside of the housing rotor is divided into multiple spaces by the vane rotor, so that multiple advancing chambers and retarding chambers are formed in the circumferential direction. When working fluid is introduced into the advancing chambers while working fluid is discharged from the retarding chambers, the rotational phase is advanced. On the other hand, when the working fluid is introduced into the retarding chambers while the working fluid is discharged from the advancing chambers, the rotational phase is retarded.
In a conventional device, for example, disclosed in Japanese Patent Publication No. 2005-344586, an advancing chamber and a retarding chamber, which are neighboring to each other in a circumferential direction, are sealed from each other by a sealing member provided at an outer peripheral wall of a vane rotor and sliding on an inner peripheral surface of a housing rotor.
However, the valve timing control device of the above prior art (JP 2005-344586) may have following problems.
When working fluid is supplied to the advancing chamber or the retarding chamber, pressure of working fluid is applied to the sealing member. Therefore, it is necessary to increase a force for pushing the sealing member to the inner peripheral surface of the housing rotor, in order to maintain a desired sealing effect even when the pressure of the working fluid is increased. However, a problem may occur due to the pressure increase of the working fluid. Namely, when the working fluid is supplied to the advancing chamber or to the retarding chamber, a rotational force is generated for rotating the vane rotor relative to the housing rotor. The sealing member receives sliding frictional force from inner peripheral surface of the housing rotor, when the vane rotor is rotated relative to the housing rotor. The sliding frictional force is increased as the pushing force for the sealing member is increased. Therefore, when the pressure of the working fluid is increased, the larger rotational force is generated against the sliding frictional force. However, when the pressure of the working fluid is decreased, a relative ratio of the sliding frictional force with respect to the rotational force becomes larger. As a result, response of rotating the vane rotor relative to the housing rotor may be remarkably decreased.
For example, when operation of an engine is stopped, such a condition occurs in which the working fluid is supplied neither to the advancing chamber nor to the retarding chamber. When the above condition continues for a long period, another problem may occur.
Namely, when the working fluid is supplied to the advancing chamber or the retarding chamber in a normal operation, liquid film of the working fluid is formed between the inner peripheral surface of the housing rotor and the sealing member. However, when the working fluid is not supplied to the advancing chamber or the retarding chamber for the long period, a condition that the liquid film of the working fluid is not formed may also continue for the long period. Then the sealing member may be fixed or adhered to the inner peripheral surface of the housing rotor.
In particular, in the valve timing control device, in which the sealing effect is obtained when the pressure of the working fluid is increased, the sealing member is more likely to be strongly fixed to the housing rotor because the force for pushing the sealing member to the inner peripheral surface of the housing rotor is increased.